1. Field of the Invention
The present invention relates to systems for testing integrated circuits (IC) and particularly it relates to probe cards for testing ICs.
2. Discussion of the Related Art
The ICs are usually manufactured in form of dice on a semiconductor material wafer. Particularly, after the manufacturing operations, the semiconductor material wafer is subdivided in dice, each including a respective IC.
Before the ICs are encapsulated and sent to the buyers, and before the ICs are installed in complex electronic systems, they have to be tested in order to evaluate their functionality, and particularly for assessing that they are not defective. During the test, it is possible to detect information regarding global or local defects (such as the presence of short circuits and undesired breakings) and more generally regarding the operation of the IC on each tested dice (for example, checking the waveforms of one or more output signals generated by the IC on each tested die). In this way it is possible to make only the dice that have fulfilled predetermined requirement perform the subsequent phases of the manufacturing process (such as the connection of the terminal wires, the packaging and the final test).
According to a known test technique, the dice including the ICs are tested before the semiconductor material wafer is subdivided in single dice. The test performed at the wafer level is denoted “Wafer Sort” or “Electrical Wafer Sort” (EWS).
In order to perform the test, a test apparatus is employed. Such test apparatus comprises a tester coupled with the semiconductor material wafer comprising the die to be tested by means of a proper probe card.
The tester is adapted to manage the signals that are used for performing the test; in the following, such signals will be denoted “test signals”. The test signals include test stimulus (for example, commands, memory location addresses, data to be written in the memory device) generated by the tester and sent to each die to be tested through the probe card, and test response signals, which are generated by the ICs integrated in each die during the test phase in response to the received test stimulus. The test response signals are sent by the IC integrated in each die to the tester through the probe card; such signals are then processed by the tester to the purpose of obtaining an indication regarding the correct/incorrect operation of the IC integrated in the dice.
In order to allow the exchange of test signals, the probe card is electrically coupled with the dice by means of proper probes. Particularly, the probe card includes of a Printed Circuit Board (PCB) connected to a plurality of mechanical probes adapted to physically contact input/output contact pads included in the dice to be tested.
Each input/output contact pad is formed by an enlarged metallization region surrounded and possibly partially covered by a passivation layer.
During the test, the contact pad is etched or scratched by the mechanical action exerted by the probe tip. In this way, it is possible to proceed to the test signals exchange between the tester and the die to be tested.
A first category of known probe cards comprises the probe cards provided with so-called cantilever probes. Such probes comprise a ring (for example made of aluminum, special alloys, or ceramic material) which is provided with an epoxidic support attached thereto. Such epoxidic support is adapted to support a plurality of test elements comprising elastic cantilever probes, made of an alloy having good electrical and mechanical properties. Particularly, each cantilever probe includes a beam having a first end connected to the epoxidic support and a second end including a tip, which in use it is directed to contact a contact pad of the die of the IC to be tested.
As an alternative to the probe cards including cantilever probes, it is possible to provide substantially vertical probes which comprise conductive wires passing through holes formed in a head of the probe card. In detail, the head of the probe card includes a top guide plate stacked on a bottom guide plate. Each probe has a tip protruding from the bottom guide plate and is adapted to electrically contacting the corresponding contact pads of the die to be tested. A contact interface known as space transformer is connected to the top guide plate and is adapted to electrically couple the probes to the PCB in such a way to allow the exchange of signals between the tester and the die to be tested.
A further type of probe card provides the use of microelectromechanical probes, or MEMS probes (wherein MEMS is the acronym of MicroElectroMechanical System). With the term of MEMS probe it is intended a probe obtainable through lithographic processes that are similar to those used for the manufacturing of the ICs. Thanks to the use of such lithographic processes it is possible to implement a great number of MEMS probes having sufficiently homogeneous structural and electrical features in a relatively economical way.
Among the various types of known MEMS probes for the use in the integrated circuits probe card field, one of the most widespread is constituted by an elastic metallic beam having an end that is connected to a substrate (for example, of a semiconductor or ceramic material) by means of one or more semiconductor support pillars, and the other end is connected to a protruding tip adapted to electrically contact the contact pads of the dice to be tested. The substrate is provided with proper conductive tracks connected to the support pillars. In this way, the exchange of test signals between the generic die and the tester through a MEMS probe may be implemented through a conductive path comprising the tip, the elastic beam, the support pillars and the conductive tracks formed in the substrate.
Further equivalent types of MEMS probes are known to those skilled in the art, such as probes constituted by a single properly shaped metallic beam that is directly connected to the substrate, probes constituted by a stringy elastic element connected to the substrate and provided with a laminar tip, probes comprising silicon beams, and probes constituted by thin bent foils made of conductive materials.
Each known MEMS probe is structured in such a way to comprise a single conductive path between the tip of the probe itself and the substrate to which it is connected. This feature complicates in a sensible way the probes' integrity verify operations. Such verify operations, required to individuate possible defective probes to be discarded and substituted, includes measurements of electric characteristics of the probes, such as the electric resistance of the probe. Since a known MEMS probe comprises a single conductive path between the tip of the probe itself and the substrate to which it is connected, the electric resistance of a MEMS probe is typically measured positioning the probe card on a conductive metallic plate in such a way the tip of the probe is in contact with such plate, and connecting a measuring device, between the metallic plate and an end of the conductive track of the substrate coupled with the probe. However, positioning the probe card on a metallic plate may be dangerous, since it is possible to damage the MEMS probes, and particularly, the tips thereof, in an irreversible way. Moreover, a measurement of such type requires the use of expensive apparatuses which have to be equipped with proper interfaces and instrumentations based on the structure of the probe card.